Knee flexor muscles | Acland's Video Atlas of Human Anatomy Skip to main content
2.2.6 Knee flexor muscles


We’ll see the adductor canal again when we look at the blood vessels. Now let’s move on and look at the muscles which produce flexion at the knee joint. We’ll revisit the main flexors, the three hamstring muscles, and two minor flexors, sartorius and gracilis.

Here are the hamstring muscles again. On the medial side, here are semi-membranosus and semi-tendinosus. As we’ve seen, they both arise from the ischial tuberosity and insert on the medial side of the knee, semimembranosus here, semitendinosus here.

On the lateral side, here’s biceps femoris. It arises both from the ischial tuberosity, and from the femur, and inserts down here, on the head of the fibula.

We’ve already seen that these three muscles, which usually act together, can produce either extension of the hip, or flexion of the knee. Whether they do one, the other, or both, is determined by what other muscles are acting in opposition to them at the time. When flexion of the knee is resisted by quadriceps, the hamstring muscles produce extension at the hip. When extension of the hip is resisted by the hip flexors, the hamstring muscles produce flexion of the knee.

In addition, the hamstring muscles, acting separately, produce medial rotation and lateral rotation at the knee joint. As we’ve seen, these movements can only happen when the knee is flexed. The two semi-muscles produce medial rotation, biceps femoris produce lateral rotation.

The other two minor knee flexors that we’ve seen already are sartorius, and gracilis. Here’s sartorius, which arises up here. Here’s gracilis, arising here. These two insert close to semitendinosus. sartorius here, gracilis here.

We’ve already seen the actions of these two muscles at the hip. At the knee, they help to produce flexion.

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